1. Field of the Invention
The invention relates to an optical system of a microlithographic projection exposure apparatus. In particular, the invention relates to an optical system of a microlithographic projection exposure apparatus which enables an increased flexibility when providing a desired polarization distribution.
2. Prior Art
Microlithography is used for producing microstructured components, such as, for example, integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus comprising an illumination device and a projection lens. In this case, the image of a mask (=reticle) illuminated via the illumination device is projected, via the projection lens, onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
During the operation of a microlithographic projection exposure apparatus there is a need to set defined illumination settings, i.e. intensity distributions in a pupil plane of the illumination device, in a targeted manner. For this purpose, besides the use of diffractive optical elements (so-called DOEs), the use of mirror arrangements is also known, e.g. from WO 2005/026843 A2. Such mirror arrangements comprise a multiplicity of micromirrors that can be set independently of one another.
Furthermore, various approaches are known for setting, in the illumination device, for the purpose of optimizing the imaging contrast, specific polarization distributions in the pupil plane and/or in the reticle in a targeted manner. In particular, it is known, both in the illumination device and in the projection lens, to set a tangential polarization distribution for high-contrast imaging. “Tangential polarization” (or “TE polarization”) is understood to mean a polarization distribution in which the planes of vibration of the electric field strength vectors of the individual linearly polarized light rays are oriented approximately perpendicularly to the radius directed toward the optical system axis. By contrast, “radial polarization” or (“TM polarization”) is understood to mean a polarization distribution in which the planes of vibration of the electric field strength vectors of the individual linearly polarized light rays are oriented approximately radially with respect to the optical system axis.
With regard to the prior art, reference is made for example to WO 2005/069081 A2, DE 10 2010 029 905 A1 and DE 10 2010 029 339 A1.